Melatonin is a hormone produced by the pineal gland and is normally secreted at night. We are able to examine the timing of the body's clock (circadian rhythm) by measuring the levels of this hormone in blood. The purpose of this study is to test the resetting effects of various doses of orally administered melatonin on human circadian rhythms in male and female subjects between the ages of 65 and 90. Investigators in this experiment have obtained synthetically derived melatonin from a pharmaceutical chemical company in the form of capsules, which will be administered orally (exogenously) during this experiment. Melatonin is an experimental drug and is not approved for general use. Previous research by our group indicates that this treatment may be able to enhance the rate at which the body can shift to a new schedule. Therefore, it may be useful for adjusting more quickly to a new work schedule or new time zones when traveling.